Whisk oil, vinegar, mustard, salt, and pepper in small bowl. Spoon over leeks.

Note: Although sometimes sold individually, leeks are usually displayed in bunches of 3 or 4. The smaller the leek, the more tender it will be. Leeks over 1 1/2 inches in diameter can be tough and woody. Choose ones with firm bright green stalks and white blemish-free bases. Avoid leeks with split or oversized bases.